Molecular dynamics simulation was applied in analyzing the material removal mechanism of the crystalline silicon substrate covered by an oxide film under the impact of a silica cluster. With the increasing of the thickness of the oxide film, the optimal film for the maximum of the number of removed atoms from the impact surface would be observed, which is due to the combinational effects of adhesions among the cluster, the oxide film and the substrate, the stability of the oxide film, and the penetration of the atoms of the substrate. Moreover, the optimal oxide film, whose thickness is about 2.5-3.0 A, is a continuous monolayer molecular structure. Furthermore, the results also showed that the contact-penetration-adhesion (CPA) material removal process, which is different from the traditional indentation-sliding process, would occur during the chemical mechanical polishing process owing to the penetration of atoms of the silicon substrate into the oxide film. These findings are instructive in understanding the material removal mechanism at atom scale during the chemical mechanical polishing process. (C) 2014 Elsevier B.V. All rights reserved.